In recent years, it has become common practice, particularly in clinical oncology, to simultaneously administer several drugs to patients for therapeutic purposes. A vast literature, both experimental and clinical, has shown that the presence of one drug may markedly influence the intensity, duration, and pharmacologic effects of another drug. Such drug interactions may have a variety of causes: they may result from alterations in drug absorption, in the binding of drugs, in tissue or to plasma proteins, which may directly or indirectly influence drug distribution, drug metabolism, or drug clearance via the bile or urine. Our major efforts are directed at understanding how drugs interact with each other and with biological organisms so that human drug therapy can be based on more rational grounds.